1. Field of the Invention
The present invention relates generally to high reflectivity reflectors, and more particularly to magnesium reflectors.
There are many optical systems in which light undergoes multiple reflections from the source or input stage to the detector or processor stage. Examples are cameras and document copiers. In these systems, it is desirable to provide reflecting surfaces having the highest possible reflectivity across the visible spectrum, i.e., from a wavelength of about 425 nanometers (nm) to 675 nm. As many of these optical systems are found in consumer products, such reflectors are preferably inexpensive and capable of maintaining their high reflectance value for several years, generally in an uncontrolled environment.
2. Description of the Prior Art
Metal Film Reflectors
The most common mirrors or reflectors are formed from thermally-evaporated films of silver or aluminum. In their freshly-deposited state, the reflectivity of these films throughout the visible spectrum is relatively high. The reflectance value for a silver reflector ranges from a low of 93.9 percent at a wavelength of 400 nm to 98.7 percent at 700 nm. For aluminum film reflectors, the value is 92.6 percent at 400 nm, falling to 90.7 percent at 650 nm and 88.8 percent at 700 nm.
If such reflective surfaces are left unprotected, their reflectivity or reflectance value decreases sooner or later due to corrosion. Aluminum forms a self limiting oxide film which causes its reflectivity to fall to a certain level between about 85 and 89 percent at 520 nm, i.e. at the middle of the visible spectrum. Thereafter, it stabilizes. Silver films will continue to corrode until they turn black. Their appearance also becomes blotchy and mottled. A reflector may be protected by depositing a film of a transparent dielectric material on the reflection surface. This, however, will reduce reflectivity.
Common household mirrors, known as second surface reflectors, are protected because they are observed through a glass sheet. Thus, they may be protected by covering the mirror's exposed side, which is not observed, with paints, lacquers, or thick films of other metals. However, even silver films used in such second surface reflectors will eventually corrode. Thus, silver mirrors are not often used for common, household applications, although they are still used for special scientific or military applications. Aluminum is now frequently used in second surface mirrors.
Partially-transmitting, silver reflector films, deposited by sputtering, are also used as low emissivity coatings or deicing coatings. These films may be completely sealed in an insulated glass window cavity or in a laminated windshield assembly. Thus, the possibility of corrosion is greatly reduced.
Multilayer Dielectric Reflectors
Very high reflectivity reflectors can be produced by the deposition of multilayer dielectric interference stacks. These stacks comprise a number of transparent films of alternating high and low refractive index (n) materials wherein each film has a one-quarter wavelength optical thickness at a particular design wavelength. Mirrors for laser resonant cavities are produced in this manner. They can have reflectivity values as high as 99.9 percent at wavelengths near the red end of the visible spectrum. Their reflectivity value decreases at longer and shorter wavelengths. For instance, for a thirteen film structure having high refractive index films of n=2.35 and low refractive index films of n=1.38, all films having a one-quarter wavelength optical thickness at 500 nm, a value of 99 percent reflectivity or greater can be maintained only over a wavelength range of approximately 12 nm.
The reflectivity range can be extended by combining two or more stacks, with the stack thicknesses adjusted so that the effective reflection regions overlap and cover the entire visible spectrum. Typically, such reflectors require 25 or more films. However, these structures are uneconomical for large area applications.
Dielectric Enhanced Reflectors
The reflectivity of metals, particularly those having a very high reflectivity, can be significantly enhanced by the addition of as few as two transparent dielectric films. In such structures, the dielectric film next to the metal film has a low refractive index while the other dielectric film has a high refractive index. See Hass, "Filmed Surfaces for Reflecting Optics", 45 J. Opt. Soc. Am. 945-52, 195-.sub.--). For example, the reflectivity of a freshly-deposited aluminum film may be increased from 91.6 to about 97 percent, at a wavelength of 550 nm, by the use of low refractive index layer, e.g. magnesium fluoride (n=l.38), and a high refractive index layer, e.g. titanium dioxide (n=2.35) or zinc sulfide. The addition of a second pair of high and low dielectric layers can boost the reflectivity value even further, e.g. to approximately 99 percent. However, the cost of the structure also increases. Further, the bandwidth of the high reflectivity zone becomes limited, as previously described. Indeed, outside the zone of high reflectivity, the reflectivity is less than that of the bare metal film. This is illustrated in FIG. 1 which shows the spectral response curves for aluminum reflectors having two (curve 10) and four (curve 12) dielectric layers. The reflectivity of bare aluminum (curve 14) is shown for comparison. The designs of the enhanced structures of FIG. 1 are shown in Table 1.
TABLE 1 ______________________________________ Refractive Optical Thickness Layer No. Index (at .lambda. = 500 nm) ______________________________________ Two Dielectric Layers 1 2.35 0.264 .lambda. 2 1.38 0.233 .lambda. 3 Aluminum Opaque (&gt; 150 nm thick) Four Dielectric Layers 1 2.35 0.255 .lambda. 2 1.38 0.271 .lambda. 3 2.35 0.261 .lambda. 4 1.38 0.229 .lambda. 5 Aluminum Opaque (&gt; 150 nm thick) ______________________________________
It is believed that most front surface enhanced reflectors include aluminum enhanced by one dielectric layer pair. Higher quality mirrors are also made that include an aluminum film enhanced by two dielectric layer pairs. One dielectric layer pair enhanced mirrors generally have a reflectivity of about 94 percent between 425 nm and 675 nm. Two dielectric layer pair enhanced mirrors have a reflectivity greater than 97 percent in the same wavelength range.
Higher reflectance values for a given enhancement can be obtained if silver is used as the base film. However, due to its cost and corrosion problems, silver-based enhanced reflectors are generally undesirable.
A more detailed description of these devices and their design techniques are given in Thin Film Optical Filters, MacLeod, 2d. ed., Ch. 4, pp. 138-46; Ch. 5, pp. 164-79 (1986).
Reflectance Values for Magnesium
The optical constants of bulk metals, including magnesium, were reported by Drude in 39 Ann. Physik. 481 (1890). The measurements are at one wavelength only, 589 nm, the sodium "D" lines. For magnesium, they show a reflectivity of 93.1 percent at that wavelength.
The reflectivity values for evaporated magnesium films in the visible spectral range were first reported by O'Bryan in 26 J. Opt. Soc. Am. 122 (1936). These values range from 68 percent at a wavelength of 405 nm to 88 percent at a wavelength of 578 nm. See also the American Institute of Physics ("AIP") Handbook, 3rd. ed., Ch. 6, pp. 124-55 (1982), which is used by many practitioners in the optical thin film art as a source of optical constants for metals. These values indicate that evaporated magnesium is unsuitable for high reflectivity reflectors. Specifically, a high reflectivity reflector should have a reflectance value of at least in excess of about 90 percent across the visible spectrum.
A general object of the present invention is to provide a reflector that has a reflectivity and transparency greater than a aluminum reflector. A more specific object of the present invention is to provide a reflector having at least one layer of magnesium and a reflectivity across the visible spectrum of at least about 92 percent.